1. Field of the Invention
The present invention relates to a light-emitting device and particularly to a light-emitting device using a light-emitting element emitting light in an ultraviolet region and at least one kind of fluorescent substance in combination.
2. Description of the Related Art
There is known a light-emitting device of the type in which a light-emitting element and at least one kind of fluorescent substance which is excited by light emitted from the light-emitting element to thereby generate fluorescence are used in combination so that light in a color different from the original color of light emitted from the light-emitting element is emitted from the light-emitting device. Particularly with the advance of the development of a light-emitting element capable of emitting light in an ultraviolet region, a trial to develop a light-emitting device capable of emitting high-luminance light has been made by using high-energy light emitted from the light-emitting element for excitation of the fluorescent substance.
For example, a light-emitting device using a light-emitting element for emitting light in an ultraviolet region and at least one kind of fluorescent substance in combination has been disclosed in Japanese Patent Publication No. 2000-208818. According to this publication, there is a proposal for a light-emitting device which is formed so that a light-emitting element for emitting light in an ultraviolet region is sealed with a silicone resin containing at least one kind of organic fluorescent substance dispersed therein.
When light in an ultraviolet region is used, it is necessary to consider particularly photo-deterioration of the member for sealing the light-emitting element. In the light-emitting device described in the publication, a silicone resin which is a material hardly deteriorated by light in an ultraviolet region is used as a sealing member to thereby prevent photo-deterioration of the sealing member.
The silicone resin is, however, low in durability against external impact because it is a soft material compared with an epoxy resin generally used as a member for sealing the light-emitting element. For this reason, both impact resistance and durability of the light-emitting element are lowered.
On the other hand, the organic fluorescent substance is used in the light-emitting device. Generally, it is thought that the organic fluorescent substance is high in light converting efficiency but is easily deteriorated by oxidization because it is highly reactive to oxygen or the like. Hence, when the organic fluorescent substance is dispersed into the sealing member uniformly as in the light-emitting device, the problem of deterioration due to oxidization arises because a part of the fluorescent substance is exposed to the outside. The deterioration of the fluorescent substance causes reduction in luminance of the light-emitting device and change in the emission color. Particularly when light in a visible region is emitted from the light-emitting element in addition to light in an ultraviolet region used for excitation of the fluorescent substance so that light obtained by mixing the light in the visible region with fluorescence generated by the fluorescent substance is radiated out, color balance is collapsed by the deterioration of the fluorescent substance so that the color of the light radiated out changes remarkably.
The surroundings of the silicone resin containing the organic fluorescent substance dispersed in the silicone resin may be coated with an epoxy resin so that both improvement in strength against external impact and prevention of deterioration of the organic fluorescent substance due to oxidization can be attained. In this case, however, an exposed portion of the fluorescent substance in the surface of the silicone resin reacts with the epoxy resin. As a result, a problem in reduction of luminance and change of the emission color caused by the deterioration of the fluorescent substance rises again.
In addition, generally, the amount of the organic fluorescent substance to be added can be hardly adjusted due to its property compared with an inorganic fluorescent substance. Hence, it is difficult to produce light-emitting devices uniform in the emission color.